The invention relates to a protective device for an electrically powered apparatus, which device is comprised of an electronic switch which responds to any conductive fluid which may find its way into the housing of the apparatus, and is further comprised of a protective switch which can be triggered by said electronic switch, to protect the operator of the apparatus from receiving an electric shock.
Examples of electrically powered apparatuses of the type described are portable hair dryers, portable air heaters (fan-forced), sunlamps, "irradiation lamps", electric shavers, etc. There is a particular hazard in using such devices if they come into contact with water. Thus, for example, a fatal accident may result if a hair dryer falls into the water in a bathroom, even if the hair dryer is not being operated.
In the state of the art, protective devices of various types are known in which, when an electrical component of the apparatus which is passing a current comes into contact with moisture, an immediate interruption of the power supply is produced by the protective device, with the aim of preventing an electric shock.
According to a first group of prior art proposals, the interruption occurs over external leakage current protection switches (located outside the apparatus and in the house wiring), or over switching relays, or simply through the circuit protection system for the house wiring itself. Thus, in Ger. OS No. 26 31 785, a whirpool bath device is described in which a measuring probe is mounted in the apparatus housing, which probe responds when moisture penetrates into the housing, and (said probe) brings about deenergization of a switching relay which is disposed outside the housing of the apparatus, e.g. in a wall plug, which relay connects the house wiring to the apparatus in a double pole switching arrangement. This known type of protective device requires special cables sic, special wall plugs, costly electronic control of the switching relay, and special measures to prevent re-energization of the relay and supply of line voltage before the precipitating hazard lit., "irregularity" has been removed.
According to a second group of prior art proposals, in response to the drawbacks of the external configuration the protective device is completely integrated into the electrically powered apparatus.
Thus, for example, it was proposed to switch in a silicon controlled rectifier (SCR) lit., "Triac" over the power supply lines of an electrical apparatus--i.e., to connect the SCR across said lines--with the three electrodes of the SCR being freely accessible in the interior of the apparatus. If the apparatus is immersed in water, the triggering electrode receives a triggering current, from ground or from one of the main electrodes (depending on the peculiarities of the installation and the phase situation in the (power supply) circuit), so that the SCR can be triggered and the power supply wires can be short circuited. This short circuiting in turn triggers the protective system for the house wiring, and shuts off power to the apparatus.
However, this arrangement violates international guidelines, which prohibit intentional short circuiting of the house wiring for the purpose of switching off an apparatus. Preconditions for trouble-free operation of the device include correct installation and fuse protection of the house wiring, and a sufficiently low impedance of the house wiring. Also, there are 220 V wiring systems having two fuse- or breaker-protected outer wires wherein only one of said wires, as a rule, is disconnected as a result of the response of a fuse or breaker protection means. In any case, the disconnection occurs simultaneously with the accident; i.e., the limitation of the electric shock to the user will depend on the time parameters of the house wiring system, and in any event a shock will not be completely prevented, since a direct connection (i.e., contact) between the water and the power supply circuit is a precondition for the production of the envisioned short circuit.
Another protective device is disclosed in Ger. OS No. 27 25 652. It involves measurement of the electrical impedance to ground of the human body; or the impedance to an outer wire of the power supply line, of said human body. For this purpose, the apparatus (appliance) has an electrically conducting contact surface, e.g., on a hand grip. This contact surface is connected to the line voltage via an electronic switch. Accordingly, if any component of this switch should fail, an additional risk would be possible.
Another precondition for proper operation is that the apparatus be connected to the house wiring in a specific manner not admitting of variation. Since most plug connectors (e.g. the "Europa" plug) do not have a set arrangement of the outer wire and the neutral wire, the unreasonable requirement is placed on the user of having to make the correct connection to the house wiring by trial and error.
Another disadvantage of this type of switch lies in the fact that the protective means also respond to a too-low impedance to ground, thus to an operating state in which there is no hazard for the user in the case of an apparatus of protective class II sic. In addition, this type of protective switch completely fails if an apparatus equipped with it comes into contact with water when it is connected with the wrong polarity to the house current; this situation was identified above as a typical cause of accidents. Under these conditions, the apparatus would not be able to operate, since the neutral wire would be disconnected, but the apparatus would still be capable of delivering a shock.